Plasma processing is known as a surface processing method such as etching or cleaning of an object to be processed such as a board on which electronic parts are mounted. In the plasma processing, predetermined surface processing is executed by placing a board as an object to be processed in a vacuum chamber which forms a processing chamber, generating plasma discharge in the processing chamber, and allowing resultant ions or electrons to act on a surface of the board. In order to stably execute the plasma processing with good processing quality, plasma discharge has to be exactly generated according to a discharge condition preset in line with a processing purpose. Consequently, various means and methods are conventionally used for the purpose of monitoring a state of generating the plasma discharge (see Patent Documents 1 to 4).
In Patent Document 1, generation of abnormal discharge and a generation position are identified by imaging the inside of a reaction room by a camera and detecting light emission by the abnormal discharge. In Patent Document 2, pulse-shaped voltage variations appearing in an output terminal of an impedance matching device used in a plasma processing device are recognized as an abnormal discharge phenomenon generation signal in plasma. In Patent Document 3, an abnormal discharge region having a possibility of generating abnormal discharge is predicted and set with respect to parameters constructing a plasma processing condition, that is, an applied high frequency output, a distance between electrodes or a pressure of the inside of a processing chamber and it is automatically determined whether or not to be the abnormal discharge region at the time of executing processing or the time of setting a condition based on setting data. In Patent Document 4, a foretaste of generation of abnormal discharge is grasped by detecting variations in ESC leakage current for electrostatic adsorption in a plasma processing device of a configuration of holding a processed body by electrostatic adsorption.